In drilling boreholes in earthen formations by the rotary method, earth-boring bits typically employ at least one rolling cone cutter, rotatably mounted thereon. The bit is secured to the lower end of a drillstring that is rotated from the surface or by downhole motors. The cutters mounted on the bit roll and slide upon the bottom of the borehole as the drillstring is rotated, thereby engaging and disintegrating the formation material. The rolling cutters are provided with teeth that are forced to penetrate and gouge the bottom of the borehole by weight from the drillstring.
As the cutters roll and slide along the bottom of the borehole, the cutters, and the shafts on which they are rotatably mounted, are subjected to large static loads from the weight on the bit, and large transient or shock loads encountered as the cutters roll and slide along the uneven surface of the bottom of the borehole. Thus, most earth-boring bits are provided with precision-formed journal bearings and bearing surfaces, as well as sealed lubrication systems to increase drilling life of bits. The lubrication systems typically are sealed to avoid lubricant loss and to prevent contamination of the bearings by foreign matter such as abrasive particles encountered in the borehole. A pressure compensator system minimizes pressure differential across the seal so that the lubricant pressure is equal to or slightly greater than the hydrostatic pressure in the annular space between the bit and the sidewall of the borehole.
A major advance in earth-boring bit seal technology occurred with the introduction of a successful rigid face seal. Rigid face seals are known in several configurations, but typically comprise at least one rigid ring, having a precision seal face ground or lapped thereon, confined in a groove near the base of the shaft on which the cutter is rotated, and an energizer member, which urges the seal face of the rigid ring into sealing engagement with a second seal face. Thus, the seal faces mate and rotate relative to each other to provide a sealing interface between the rolling cutter and the shaft on which it is mounted.
The combination of the energizer member and rigid ring permits the seal assembly to move slightly to minimize pressure fluctuations in the lubricant, and to prevent extrusion of the energizer past the cutter and bearing shaft, which can result in sudden and almost total lubricant loss. U.S. Pat. No. 4,516,641, to Burr; U.S. Pat. No. 4,666,001, to Burr; U.S. Pat. No. 4,753,304, to Kelly; U.S. Pat. No. 4,923,020 to Kelly; and U.S. Pat. No. 6,142,249 to Zahradnik et al are examples of rigid face seals for use in earth-boring bits. Rigid face seals substantially improve the drilling life of earth-boring bits of the rolling cutter variety. Earth-boring bits with rigid face seals frequently retain lubricant and thus operate efficiently longer than prior-art bits.
Because the seal faces of rigid face seals are in constant contact and slide relative to each other, the dominant mode of failure of the seals is wear. Eventually, the seal faces become pitted and the coefficient of friction between the seal faces increases, leading to increased operating temperatures, reduction in seal efficiency, and eventual seal failure, which ultimately result in bit failure. In an effort to minimize seal wear, seal rings of prior-art rigid face seals are constructed of tool steels such as 440C stainless steel, or hardenable alloys such as STELLITE. Use of these materials in rigid face seals lengthens the drilling life of bits, but leaves room for improvement of the drilling longevity of rigid face seals, and thus earth-boring bits.
Very hard, wear-resistant layers and coatings have been developed in general, such as those employing diamond. These coatings, however, generally need to be applied at high temperatures and high pressures. The coatings are applied after the steel ring has been hardened. If the high temperatures exceed the lowest transformation temperature of the steel of the ring, such as the temperature at which the steel ring has been tempered, this would adversely affect the properties of the seal ring.
U.S. Pat. No. 6,209,185 to Scott discloses applying a diamond layer to substrate, then attaching the diamond layer to the rigid ring. This avoids having to heat the hardened ring beyond its lowest transformation temperature, but it does require attachment by brazing, epoxy or the like. U.S. Pat. No. 6,045,029 to Scott discloses forming a diamond layer directly on a rigid seal ring by a process that is accomplished at a temperature lower than the lowest transformation temperature of the metal of the seal ring. This may be done in an amorphic diamond process or by forming the diamond layer separately and attaching it to the rigid ring of the seal.